1. Field of the Invention
The present invention relates to a method of producing an electroluminescence (hereinafter referred to as xe2x80x9cELxe2x80x9d) display device.
2. Description of Related Art
EL display devices using an EL element have recently attracted interest as potential replacements for devices such as CRT or LCD displays.
Also, EL display devices having a thin film transistor (TFT) as a switching element for driving the EL element have been studied and developed.
FIG. 1 shows, in a plan view, one display pixel of an organic EL display device. FIG. 2 is an equivalent circuit diagram corresponding to one display pixel of an EL display device.
Referring to FIGS. 1 and 2, a display pixel is formed in a region enclosed by a gate signal line 51 and a drain signal line 52. Around the intersection of both signal lines is formed a first TFT 30 as a switching TFT. A source 31s of the first TFT 30 also functions as a storage capacitor electrode 55, and a storage capacitor 70 is formed between the source 31s and a storage capacitor electrode 54 which will be described later. The source 13s of the TFT 30 is also connected to a gate 41 of a second TFT 40 which drives an organic EL element. A source 43s of the second TFT 40 is connected to an anode 61 of the organic EL element while a drain 43d is connected to a power source line 53 for driving the organic EL element.
The storage capacitor electrode 54 is disposed in parallel to the gate signal line 51 so as to run through each pixel. The storage capacitor electrode 54 is composed of chromium or the like, and charges are accumulated between the storage capacitor electrode 54 and the capacitor electrode 55, which also functions as the source 31s of the first TFT 30, via a gate insulating film 12. The storage capacitor 70 is provided so as to hold a voltage to be applied to the gate 41 of the second TFT 40.
On a substrate composed of a glass or the like are formed the above-mentioned first and second TFTs 30, 40, the lines 51, 52, 53 for supplying a gate signal, a data signal, and an element driving power to these TFTs, and the above-referenced storage capacitor electrode 54. A planarizing insulating film 17 is further formed so as to cover these elements. Over the planarizing insulating film 17, a transparent electrode using ITO (Indium Tin Oxide), specifically, an anode 61 of an organic EL element 60, is disposed.
The organic EL element 60 comprises the anode 61, an emissive element layer comprising an organic compound and formed on the anode 61, and a cathode formed on the emissive element layer as a common layer for each element. The emissive element layer comprises at least an emissive layer, and may have a laminated structure including, for example, a hole transport layer, an emissive layer, and an electron transport layer, which are disposed in that order from the anode side.
In the organic EL element configured as described above, holes injected from the anode and electrons injected from the cathode are recombined inside the emissive layer to excite organic molecules forming the emissive layer for causing exciton. In the process of radiation and deactivation by the exciton, the emissive layer produces light which is emitted from the transparent anode through the transparent insulating substrate.
Next, a method of forming an emissive element layer of the above-described organic EL element will be described.
For a color display device, in order to emit red (R), green (G), and blue (B) light, it is necessary that the emissive layers for emitting these colors are formed from different materials associated with colors to be emitted. The emissive material layers are formed on the hole transport layer by means of evaporation. More specifically, an emissive material of each of different colors including, for example, red, green, and blue is sequentially evaporated on the corresponding anode 61 in an island pattern corresponding to the anode 61.
Thus, the emissive layer for emitting each color of R, G, or B is sequentially formed corresponding to the anode 61 for each pixel electrode in a repeated manner, so that, when observed in a plan view, the emissive layers are arranged within the display area of the substrate in a matrix pattern.
When forming the emissive layers, a metal mask 250 having openings at locations corresponding to the display pixels for emitting light of the same color, as shown in FIG. 3, is moved in the right or left direction in FIG. 3, so that an emissive layer material for each color is evaporated.
FIG. 3 shows a case of evaporating an emissive material of B, with the emissive layers of R and G already formed. Namely, an emissive material of blue, which is placed on an evaporation source 200, is evaporated and accumulated at locations corresponding to the display electrodes for the B color.
However, the metal mask 250 for use in evaporation as shown in FIG. 3, which has openings 251 at locations corresponding to the emissive layers of the same color, causes problems such as the following. When the thickness of the metal mask is h, and the width of the opening (the horizontal direction in FIG. 3) is d, and the width d is too large with regard to the thickness h, the emissive material evaporated from the evaporation source is deposited not only on the intended display pixel, but spreads to areas of the emissive layer of an adjoining pixel electrode designed to emit a different color. For example, on a pixel electrode in which the emissive material of G is already evaporated and accumulated, emissive material for B color for the adjacent pixel can be mixed such that a mixed color of B and G is generated. This leads to a disadvantage that intended color display can not be obtained.
The present invention was conceived in view of the aforementioned problems of the related art and aims to provide a method of producing an EL display device in which spread of an emissive layer material onto an adjoining pixel electrode is prevented, thereby reducing color mixture and enabling more accurate and consistent display of desired colors.
In accordance with the present invention, there is provided a method of producing an electroluminescence display device comprising an emissive layer for emitting a color formed between an anode and a cathode constituting a pixel electrode, in which provided on said anode to form said emissive layer is a mask having a thickness h and an opening width d determined between adjacent display pixels such that the thickness h and the opening width d satisfying the relationship h greater than nxc3x97d, wherein n greater than 1.
Further, in the above method, it may be preferable that the thickness h and the opening width d of the mask satisfy a relation of h greater than nxc3x97d where 1 less than nxe2x89xa62.5.
Further, in the above method, said mask may be formed from a metal or semiconductor.
As described above, in accordance with the present invention, the emissive layer is formed using a mask having an appropriate thickness with respect to the width of the opening, so that mixture of colors between emissive layers in adjoining pixel electrodes can be reduced. It is therefore possible to provide an EL display device capable of displaying pure colors.